Velocity modulation tube



3 L l v .2 W 6 TF lvv 0 Nu Q 7 E N 1 v VK R 6, H W Mm 2 r 0 2 C .7 x w \f 6 a f w M F u v Q J. KEUFFEL VELOCITY MODULATION TUBE Filed Oct. 19, 1945 Nbv; 11, I952 Patented Nov. 11, 1952 VELO CITY ,MODULATION TUBE Jack W. Keuffel, Cambridge, Mass., assignor to the United States of America as represented by the Secretary of War Application October 19, 1945, Serial No. 623,418

This invention relates generally to electrical apparatus and more particularly to velocitymodulation electronic tubes.

In the operation of velocity-modulation tubes as high frequency oscillators or amplifiers it is many times desirable to alter the operating frequency. Generally, small frequency variation in such tubes are obtained by deformation of the resonant cavities associated with them. However, the variation of frequency obtainable by this means alone is limited and when a frequency variation beyond this range is desired, it is necessary to shift the electrical mode of operation such as may be obtained by completely changing the basic configuration of the associated frequency determining cavity or cavities. In velocitymodulation tubes which have the frequencydetermining cavities external tothe seal enclosing the evacuated portion of the tube, the usual procedure has been to remove the external cavity and. replace it by a different type of cavity and thus make such tubes readily adaptable for operation over a wide range of frequencies.

It is an object of this invention to provide a tuning cavity for a two-gap, velocity-modulation, focused-beam oscillator tube in which the operating frequency is changed by reducing the drift tube transit time.

It is a further object of this invention to provide a means of reducing the drift tub transit time in a two-gap, magnetically-focused, velocity-modulation oscillator tube by employing a coaxial type cavity that will function when the drift tube transit time is reduced to a one-quarter cycle, thus permitting the operation of a high frequency velocity-modulation tube at frequencies other than thos for which the tube was originally designed.

The requirement necessary to accomplish the above objects is met by replacing the usual waveguide type cavity associated with velocity-modulation tubes by a coaxial line type cavity.

Other objects, features, and advantages of this invention will suggest themselves to those skilled in-the art and will become apparent from the following description of the invention taken in connection with the accompanying drawings in which:

Fig. 1 is a diagrammatic representation of a tube and its associated cavity employing the principles of this invention;

Fig. 2 is an elevation showing the general physical configuration of th tube shown in Fig. 1; and

Fig. 3 is an elevation of the assembled tube and cavity shown in Fig. 1.

Referring now to Fig, 1, a velocity-modulation tube II), including a glass envelope I I, a collector electrode I2, a cathode I3, a drift tube I4, buncher 7 grid I5, and catcher grid I 6, is connected to a 2 Claims. (01'. 315-6) coaxial cavity l1, l1, consisting of inner conductor I8, I8 outer conductor I9, I9, and tuning plunger 20, 20. The physical configuration'of the tube without the cavities attached is shown in more detail in Fig. 2. There the configuration of the metallic drift tube I4 is shown cen tered between grids I5, and I6.

Reference to Fig. 3 shows tube It assembled with coaxial cavity IT. A hollow box-like structure 2| provides means for holding tube In and coaxial cavity I! in their proper assembled position so that the outer conductor I9 of coaxial cavity l I is in electrical contact with the portions of both buncher disc I5 and catcher disc I6 which extend outside the glass envelope I I. Inner conductor I8 of coaxial cavity I1 is also held by means of structure 2| in contact with a portion of drift tube I 4, which extends outside of glass envelope II.

The usual circuit involving the use of a velocity-modulation tube of this general type operates with an associated frequency determining wave-guide type cavity of either cylindrical or rectangular shape. With a Waveguide type external cavity connected to grids I5 and I6, the drift tube I4 is grounded and the tube with its associated frequency determining cavity operates at a frequency corresponding to the relation 0 is the electron bunch transit time in cycles, and N is any positive integer or zero.

With the arrangement of tube and cavity herein disclosed oscillations will occur when the electron bunch transit time follows the relation This is due in part to the fact that the drift tube, instead of being grounded, is connected to the inner conductor of the coaxial cavity.

Adjustable short circuiting plungers 2U, 20 are provided at each end of cavity I! to vary the operating frequency for operation over the range with the electrical mode above defined.

With this arrangement herein disclosed, electrons passing the buncher grid I5 when it is at the maximum positive potential, proceed through drift tube I4, which is negative with respect to the buncher grid l5, and enter an intense radio frequency field between drift tube I4 and catcher grid I6 in which drift tube I4 is also negative with respect to catcher grid I 6. Since the transit time of the electron bunches through drift space tube I4 follows the relation 0=N+ A electron bunches will move through a retarding field between drift space tube l4 and the catcher grid Hi, to deliver energy to the coaxial cavities and reinforce oscillations therein.

Thus, this arrangement of circuit elements permits the drift space transit time to follow the relation 0=N+ A rather than 0=N enabling a two-gap, velocity-modulation type tube to be operated over adifferent frequency range.

While there has been here described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes andmodifications may be made therein withoutdeparting from the scope of the invention.

What is claimed is:

1. In combination, a velocity modulation tube for producing oscillations at a very high frequency, comprising an electron emitting member,

a buncher grid, a drift tube, a catcher grid, and a. collector electrode, arranged in the order named, all of the foregoing elements of said tube havinggconnections: e-xternalthereto, a two-part coaxialtransmissionline disposed so that its axis is perpendicular to the axis of said tube, said tube being disposed between-said parts, the outer conductor of said line being in contact with the external connections of said buncher: and catcher grids, the inner'conductor of said line being connected to the external connection of said drift tube, and apair of means for respectively shortcircuitingeaoh part of said line, the transit time of the electrons in said drift tube incycles being equal to N where Nirepresents zero or any positive integer.

2. Invcombination, a velocity modulation tube for producing oscillations at a very highVfre-V quency, comprising an electron emitting member. a buncher grid, a drift tube, a catcher grid, anda collector electrode, positioned in the order named, all of the foregoing elements of said tube having connectionsexternal thereto, atwo-part coaxial transmissionlinedisposed-so that its axis is perpendicular to the axis of said tube, said tube being disposed between said parts, the outer conductor of. said line being in contact with the external connections of said buncher and catcher grids, theqinner' conductor of said line being connected to.the: external:connection of said drift tube, and a pair of plunger-s for variably short-circuiting the respective parts of said coaxial line, the transit time of the electrons in said drift tube in cyclesibeing equal to N+ A where N represents zero or any positive integer.

JACK W. KEUFFEL.

REFERENCES CITED The following. references are-of record in the file of'tliis'patent:

UNITED STATES PATENTS Number Name Date 2,153,728 Southworth Apr. 11, 1939 2,200,986 Fraenckel May 14, 1940 2,222,902 Hahn Nov. 26, 1940 2,235,527 McArthur Mar. 18, 1941 2,284,405 McArthur May 26, 1942 2,287,845 Varian et a1; June 30, 1942 2,289,846. Litton- July 14, 1942 2,368,031 Llewellyn Jan. 23, 1945 2,411,538 Goodchild Nov. 26, 1946 2,462,085 Fremlin Feb, 22, 1949 

